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@@ -0,0 +1,63 @@
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+#lang racket
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+
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+(require "../lib/utils.rkt")
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+
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+(define lines
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+ (call-with-input-file "input"
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+ (λ (fp) (get-lines fp))))
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+
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+(define mat (list->vector lines))
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+
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+(define (char-at x y)
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+ (string-ref (vector-ref mat y) x))
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+
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+(define (set-mat! x y c)
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+ (string-set! (vector-ref mat y) x c))
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+
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+(define (move-stone x1 y1 x2 y2)
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+ (define t (char-at x1 y1))
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+ (set-mat! x1 y1 (char-at x2 y2))
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+ (set-mat! x2 y2 t))
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+
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+(define (find-new-pos x y)
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+ (let loop ((new-y y))
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+ (if (or (= new-y 0)
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+ (not (char=? #\. (char-at x (- new-y 1)))))
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+ new-y
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+ (loop (- new-y 1)))))
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+
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+(define height (vector-length mat))
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+(define width (string-length (vector-ref mat 0)))
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+
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+(define (tilt)
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+ (let loop ((y 0))
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+ (if (>= y height)
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+ (void)
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+ (let ()
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+ (let loop1 ((x 0))
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+ (if (>= x width)
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+ (void)
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+ (let ()
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+ (define c (char-at x y))
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+ (if (char=? #\O c)
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+ (let ()
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+ (define new-y (find-new-pos x y))
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+ (move-stone x y x new-y)
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+ (loop1 (+ 1 x)))
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+ (loop1 (+ 1 x))))))
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+ (loop (+ 1 y))))))
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+
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+(define (count)
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+ (let loop ((x 0) (y 0) (sum 0))
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+ (if (>= y height)
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+ sum
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+ (if (>= x width)
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+ (loop 0 (+ 1 y) sum)
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+ (let ()
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+ (define c (char-at x y))
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+ (if (char=? c #\O)
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+ (loop (+ 1 x) y (+ sum (- height y)))
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+ (loop (+ 1 x) y sum)))))))
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+
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+(tilt)
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+(count)
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